The present invention relates to a power unit for driving a magnetron in which cooling of the power unit for driving a magnetron is improved in the technical field of a high frequency heating apparatus, such as a microwave oven, for dielectric heating using a magnetron. Furthermore, the present invention relates to the cooling configuration of a plurality of heat sinks assembled on a printed circuit board, wherein power semiconductor devices with non-insulation packages which are located at electric potential differences are mounted on respective said heat sinks.
Conventionally, as shown in FIG. 4, a switching power source used as a power unit for driving a magnetron is such that the semiconductor switching element 1 is attached to a heat radiating fin 2, which is made of aluminum, by a screw 3 in order to prevent the elements from being damaged due to a temperature rise resulting from switching losses of the semiconductor switching element 1, and heat that is generated due to switching losses are thermally transmitted to the heat radiating fin, whereby the heat is radiated from the heat radiating fin 2. Further, in order to efficiently thermally conduct switching losses, which are generated by the semiconductor switching elements 1, to the heat radiating fin 2, the semiconductor switching elements 1 have a collector part exposed on the rear side thereof, a thermal conducting filler having good thermal conductivity is coated and filled between the rear side thereof and the heat radiating fin, and the collector part 4 is brought into contact with the heat radiating fin 2, thereby having improved thermal conduction.
However, a high voltage-resisting semiconductor switching element for a power unit for driving a magnetron has been requested in line with high output of a high frequency heating apparatus. Since the yields of high voltage-resisting semiconductor switching elements are not sufficient and these are expensive, there is a problem in procurement thereof. Therefore, for a power unit for driving a magnetron, circuits that are composed of two versatile medium voltage-resisting semiconductor switching elements connected in series have been used as shown in FIG. 5. That is, the commercially available power supply 5 is rectified to be a direct current by a rectification part 6 and is made into high frequency by a switching portion 9 composed of semiconductor switching elements 7 and 8. The high frequency is boosted by a boosting transformer 10, and is subjected to a voltage doubler rectifier by a high-voltage doubler rectifier circuit part 11. Then, the same is provided to a magnetron 12.
However, in a construction in which semiconductor switching elements are attached to a conventional heat radiating fin, although two semiconductor switching elements 7 and 8 are attached to one heat radiating fin 2, in view of the circuit, a collector terminal part 13 of one semiconductor switching element 7 is connected to an emitter terminal part 14 of the other semiconductor switching element 8, and these are to be held in the same potential. On the other hand, in order to efficiently thermally conduct the switching losses of the semiconductor switching elements 7 and 8 to the heat radiating fin, the collector terminal part 13 of the semiconductor switching element 7 and the collector part of the same potential are exposed on the rear side of the semiconductor switching elements, a thermal conducting filler of good thermal conductivity is coated thereon, and these are brought into contact with the heat radiating fin. However, where the two semiconductor switching elements having the collector parts thereof exposed on the rear side, where thermal conductivity is improved, are attached to one heat radiating fin as they are, the collector parts are made into the same potential, wherein the circuit shown in FIG. 5 cannot be configured.
Therefore, as shown in FIG. 6, it was necessary that the heat radiating fin was divided into heat radiating fins 15 and 16, wherein the semiconductor switching elements 19 and 20 having collector parts 17 and 18 exposed on the rear side thereof were, respectively, attached thereto by screws 21 and 22, and two heat radiating fins were electrically insulated from each other, and the heat radiating fins were not held in the same potential. Or as shown in FIG. 7, in order to attach two semiconductor switching elements 24 and 25 to a single heat radiating fin 23, a semiconductor switching element 24 having a larger switching loss had a collector part 26 exposed on the rear side thereof and the same was attached to the heat radiating fin 23 by a screw 27 while the other semiconductor switching element 25 was electrically insulated with a resin armoring the outside of the collector part on the rear side thereof and was attached to the heat radiating fin 23 with a screw 28.
Thus, since in the former two heat radiating fins are required, and it is necessary that these two heat radiating fins are electrically insulated from each other, there is a problem in that a disadvantage is brought about with respect to the installation plane of the heat radiating fin in view of constituting a power unit for driving a magnetron.
Also, since one of the semiconductor switching elements has the collector part on the rear side thereof electrically insulated with an armoring resin although in the latter the semiconductor switching elements can be attached to a single heat radiating fin, it is difficult to conduct heat resulting from switching losses to the heat radiating fin in view of thermal conductivity, wherein it is necessary to make the heat radiating fin larger in order to secure a sufficient cooling effect or it is necessary to make large a cooling fan for cooling the heat radiating fin. Therefore, there is another problem in that a disadvantage occurs in view of the plane of installation of the heat radiating fin and cooling fan when constituting a power unit for driving a magnetron.
As another method, when attaching, to the heat radiating fin, one of the two semiconductor switching elements having the collector part thereof exposed on the rear side thereof, in which heat conductivity has been improved, a silicon sheet or mica plate having an insulative property and thermal conductivity is caused to intervene and is attached between the semiconductor switching element having the collector part exposed on the rear side thereof and the heat radiating fin 7. However, since the thermal conductivity of the silicon sheet and mica plate is 1.0 through 1.5xc3x9710xe2x88x923 cal/cm.sec.K, and the thickness thereof is 0.3 through 1.0 mm, it is difficult to conduct heat, which is generated by the semiconductor switching elements, to the heat radiating fin, problems occur in that a necessity of making the heat radiating fin large arises in order to sufficiently cool the semiconductor switching elements, and a cooling fan for cooling the heat radiating fin is made large.
Further, in the prior art of such a heat sink unit for assembly on a printed circuit board, heat sinks at different potentials have been separately attached to a printed circuit board by screws or the like. Alternatively, an insulation sheet has been used for one of the semiconductor devices. FIG. 11 is an assembly diagram showing a prior art heat sink unit for assembly on a printed circuit board in which heat sinks are separately attached to a printed circuit board. FIG. 12 is a wiring print diagram of the printed circuit board.
In FIG. 11, a printed circuit board 1 is provided, thereon, with a first heat sink 103 to which a first power semiconductor device 102 is attached by a screw, and similarly with a second heat sink 105 which is separated therefrom by a predetermined insulation distance and to which a second power semiconductor device 104 is attached by a screw.
As shown in FIG. 12, each heat sink is attached to the printed circuit board 1 by a screw. The wiring print pattern 107 around each screw 106 is separated from the screw 106 by a predetermined insulation distance.
Nevertheless, the prior art has a first design-relevant problem wherein the necessity of insulation separates the heat sinks 103 and 105 into two distinct components. A second problem is that the necessity of the separation and the insulation distance reduces the surface area of the heat sinks 103 and 105 on condition that the arrangement in the printed circuit board 101 is maintained unchanged. A third problem is that the heat sinks 103 and 105 need to be independently attached to the printed circuit board 101. These problems have caused a first difficulty in that the number of assembly processes is increased, and that thereby the workability is reduced. A second difficulty has been that on condition that the arrangement in the printed circuit board 101 is maintained unchanged, the reduction in the heat radiation area reduces the heat radiation efficiency, and that thereby the heat design becomes difficult. A third difficulty has been that the necessity of the insulation distance between the screw 106 and the wiring print pattern restricts the area of the wiring print pattern for large currents. In order to avoid this, the pattern had to be supplemented by lead wires or the like in some cases.
In a case where two power semiconductor devices with non-insulation package are assembled on a single heat sink by means of an insulation sheet, the above-mentioned problems in the number of components and the area of the wiring print pattern are resolved. Nevertheless, a much higher heat resistance of the insulation sheet than that of the non-insulation package causes a large difficulty in heat design.
Furthermore, conventionally, as such another type of switching power supply for a high frequency heating apparatus, for example, those shown in FIG. 15 and FIG. 16 have been generally used. An alternating current of commercial power supply 201 is converted into a DC voltage by rectifier 202, and in response to this DC voltage, inverter circuit 205 generates a high frequency voltage to the primary winding of high voltage transformer 206 by turning switching devices 203 and 204 ON and OFF, and the high voltage transformer 206 excites a high frequency high voltage to the secondary winding. This high frequency high voltage is rectified into a DC high voltage by high voltage rectifying circuit 207, and applied to magnetron 208. The magnetron 208 is driven in response to this DC high voltage, and generates a radio wave of 2.45 GHz.
By the abovementioned operation, the rectifier 202 generates an approximately 15 through 25W loss and the semiconductor switching devices 203 and 204 generate an approximately 30 through 50W loss each. Therefore, for cooling, the rectifier 202 and semiconductor switching devices 203 and 204 are attached to radiation fin 209.
However, in the abovementioned conventional construction, since the semiconductor switching devices 203 and 204 generate a loss twice as much as that of the rectifier 202, in proportion to the loss, each junction temperature of the semiconductor switching devices 203 and 204 naturally becomes much higher than that of the rectifier 202.
Furthermore, since the rectifier 202 and semiconductor switching devices 203 and 204 are attached to the same radiation fin 209, the semiconductor switching devices 203 and 204 receive heat from the rectifier 202 due to heat conduction, and each junction temperature of the semiconductor switching devices 203 and 204 rises further, and sometimes exceeds a temperature that is permissible in terms of reliability. The rectifier 202 also receives heat from the semiconductor switching devices 203 and 204 due to heat conduction, and the junction temperature of the rectifier 202 also rises, however, the loss of the rectifier is originally small, so that the temperature is lower than the temperature that is permissible in terms of reliability. Therefore, in order to restrict each junction temperature of the semiconductor switching devices 203 and 204 within the temperature that is permissible in terms of reliability, semiconductor switching devices 203 and 204 that are expensive although small in loss, and radiation fin 9 with higher cooling performance have been conventionally used, whereby the costs have been high.
To solve such a problem, as shown in FIG. 17, a construction in which the radiation fin is divided into a radiation fin 209a for the rectifier 202 and a radiation fin 209b for the semiconductor switching devices 203 and 204 has been proposed. This case is advantageous since the semiconductor switching devices 203 and 204 do not receive heat from the rectifier 202 and the junction temperature is accordingly suppressed from rising. However, in this construction, the radiation fin is divided into two, the number of assembly processes for manufacturing a switching power supply for a high frequency heating apparatus becomes twice as much, the manufacturing costs increase, so that costs are high all the same.
The present invention solves these conventional and other problems. It is therefore a first object of the invention to provide a power unit for driving a magnetron, wherein semiconductor switching elements are able to be attached to a single heat radiating fin where a power unit for driving a magnetron is composed of semiconductor switching elements connected to each other in series, two semiconductor switching elements having a collector part having good thermal conductivity to the heat radiating fin and exposed on the rear side thereof are used, the collector part of one of the semiconductor switching elements is electrically insulated from the heat radiating fin with a simplified construction, and heat which is produced by the semiconductor switching elements is radiated by the heat radiating fin at better thermal conductivity.
Further, a second object of the invention is to provide a heat sink unit for assembly on a printed circuit board capable of reducing the number of assembly processes, permitting easy design in the heat radiation of the power semiconductor devices, facilitating the area of the wiring print pattern, and thereby improving the accommodation reliability.
Furthermore, a third object thereof is to provide a switching power supply for a high frequency heating apparatus, which can restrict each junction temperature of semiconductor switching devices within a temperature that is permissible in terms of reliability by a simple and inexpensive structure.
According to a first aspect of the invention, a power unit for driving a magnetron is composed so that the same includes semiconductor switching elements, a heat radiating fin, a spacer, and a thermal conducting filler, wherein said semiconductor switching elements have a collector part exposed on the rear side thereof, the exposed collector part of one semiconductor switching element is coated and filled with said thermal conducting filler via said spacer and is attached to said heat radiating fin, the exposed collector part of another semiconductor switching element is directly coated with said thermal conducting filler with no space intervened, and is attached to the same heat radiating fin, and further the two semiconductor switching elements are connected to each other in series, whereby since the exposed collector part of one semiconductor switching element is electrically insulated from the heat radiating fin by the spacer, and a thermal conducting filler is coated on and filled in the collector part, it becomes possible to effectively conduct heat resulting from switching losses to the heat radiating fin in view of thermal conductivity. Further, since a single heat radiating fin is sufficient in constructing the power unit for driving a magnetron, and heat resulting from switching losses can be effectively transmitted to the heat radiating fin, a cooling fan can be made small.
Preferably, the power unit for driving a magnetron may be constructed so that the two semiconductor switching elements have a collector part exposed thereon, a spacer projecting from the plane of the collector part is constituted on one collector part side thereof, and a thermal conducting filler is coated and filled in an air gap between the collector part of said semiconductor switching elements and the heat radiating fin, whereby since the collector part is electrically insulated from the heat radiating fin in spacing secured by the projecting spacer and a thermal conducting filler is coated on and filled in the collector part, heat resulting from switching losses can be effectively transmitted to the heat radiating fin in view of thermal conductivity. Further, since a single heat radiating fin is sufficient in constructing the power unit for driving a magnetron, and heat resulting from switching losses can be effectively transmitted to the heat radiating fin, the cooling fan can be made small.
Further, preferably, the power unit for driving a magnetron is constructed so that two or more holes are provided in a spacer whose thermal conductivity is 0.5 through 1.5xc3x9710xe2x88x923 cal/cm.sec.K, and a thermal conducting filler is coated and filled in said holes of the spacer, whereby since a thermal conducting filler is provided into holes secured at the spacer, and heat resulting from switching losses can be effectively transmitted to the heat radiating fin, a single heat radiating fin is sufficient in constructing the power unit for driving a magnetron, and the heat resulting from switching losses can be effectively transmitted to the heat radiating fin, wherein the cooling fan can be made small.
According to the second aspect of the invention, in a heat sink unit for assembly on a printed circuit board, a plurality of heat sinks assembled on a printed circuit board are thermally linked to each other by an insulator having thermal conductivity.
By virtue of this, the heat sinks constitute an apparently single structure, and the heat sinks are thermally linked to each other, whereby the heat radiation efficiency is improved. Further, the area of the wiring print pattern is facilitated.
A plurality of heat sinks assembled on a printed circuit board are thermally linked to each other by an insulator having thermal conductivity. By virtue of this, the heat sinks constitute an apparently single structure, and the heat sinks are thermally linked to each other by the insulator having thermal conductivity. Accordingly, the assembly workability is improved. Further, the heat radiation efficiency is improved.
Preferably, in the heat sink unit for assembly on a printed circuit board, the power semiconductor devices and the heat sinks are attached simultaneously together with the insulator having thermal conductivity by screws or the like. Accordingly, the heat sinks are securely fixed to each other by the insulator.
Preferably, in the heat sink unit for assembly on a printed circuit board, the insulator having thermal conductivity is composed of a metallic material covered with a thin insulator film. Accordingly, the thermal linkage between the heat sinks is improved drastically. This permits easy design in the heat radiation.
Preferably, in the heat sink unit for assembly on a printed circuit board, the heat sinks constitute an apparent single heat sink, whereby the number of screw attachments is reduced between the printed circuit board and the heat sink unit. This facilitates the area of the wiring print pattern, and thereby improves the reliability.
According to third aspect of the invention, a switching power supply for a high frequency heating apparatus comprises a rectifier for rectifying a commercial power supply, at least one semiconductor switching device for switching a rectified output rectified by the rectifier, and a radiation fin for cooling the rectifier and semiconductor switching device, and is constructed so that the attaching position of the rectifier to the radiation fin is set so that the outer shape of the rectifier package protrudes from the outer shape of the radiation fin.
Thereby, the contact area between the rectifier and radiation fin is reduced, and heat quantity conducted from the rectifier to the semiconductor switching device is reduced. Therefore, without using an expensive semiconductor switching device with a lower loss and without using a number of radiation fins, an inexpensive switching power supply for a high frequency heating apparatus which can restrict the junction temperature of the semiconductor switching device within a temperature that is permissible in terms of reliability by use of one radiation fin can be provided.